1. Field of the Invention
The invention pertains to the art of high frequency radiometry and to radiometric devices for seeking substantially stationary and other targets against the high background noise; it more particularly relates to radiometric devices adapted to recognize special characteristics of substantially point targets, both moving and stationary, even though immersed in such noisy environments, by the employment of novel adaptive automatic gain control arrangements for separating the desired target signals from such significant background noise.
2. Description of the Prior Art
Radiometric target searching and tracking systems are well known in the art and are so generally characterized, by the inherent nature of their operating environment, to be of specialized utility. For example, they have not generally been totally satisfactory for use in high noise level environments, as in detectiang small metal objects like fixed navigation markers or other metal objects such as parked vehicles in noisy environments. A typical example of prior dual-mode radiometric systems is that of the Lararchik et al U.S. Pat. No. 3,787,857 for a "Dual Mode Radiometric System", issued Jan. 22, 1974 and assigned to Sperry Rand Corporation. The Lazarchik et al system features selective operation in a passive mode or in an active mode, as does the present invention, the active mode providing a mechanism for the deliberate illumination of the selected target for improving the detected radiometric temperature contrast between the target and its spatial background and thereby for increasing the range at which such a target may first be detected.
There are basic problems associated with such radiometric systems when employed in searching, acquiring, and automatically tracking relatively small metal objects such as trucks or other vehicles whose high frequency energy-radiating properties generally vary over a wide range. In particular, the targets involved are often small with respect to the angular region subtended by the circularly symmetric antenna receptivity pattern. For example, at the normal acquisition range between the target and the radiometric system, target size may easily be an order of magnitude smaller than the cross section of the antenna receptivity pattern where the latter intercepts the earth. If the noise from the background surrounding the target were ideally homogeneous and constant, it would be possible to detect even a point target; but the background inherently fluctuates. Such wide fluctuations have a relatively low frequency spectrum and additionally tend to mask the signals from small targets and to make their detection difficult.
Thus, a major problem element for any air-to-ground active-passive radiometric tracking system in which antenna aperture size is constrained is that the received signal from a target is small in relation to the unwanted background signals especially at maximum detection and tracking slant ranges. This is understood in part from FIGS. 1 and 2 that show the received power in an active mode at radiometer 6 as a function of conical scan beam position 1, 2, 3, or 4. As noted, the power received from the background is present throughout all beam positions and is significant with respect to the power from a target 5 appearing at beam position 3. The tracking system must be able to separate both stationary and moving target signals from the background clutter and to provide steering signals that are true indications of angular pointing error. FIG. 3 illustrates the typical received power contributions from the background and target 5 as a function of range for a 35 GHz active mode seeker with a five inch diameter antenna. As will be noted, at a range of 5,000 feet or greater, the received background power is at least ten times greater than the received target power. At a range of 1,500 feet, the received powers from the two sources become equal; at less than 1,500 feet, the target power exceeds the background power. The automatic gain control system for the tracker must be capable of separating the target power from the background power and must prevent receiver saturation with increasing signal levels. In addition, the gain control must preserve a substantially fixed scale factor between percent modulation at the conical scan frequency and angular error, independent of received signal strength. If this is not done, loop gain of the tracking system increases as the craft descends and the tracking system becomes unstable and will probably lose the target.
There are, of course, gain control methods available in the fully active radar sytem art, but these are generally not adaptable to the active-passive radiometer tracker for several reasons; for example, range and gain control gating systems cannot operate without the synchronized type of operation characteristic of the active mode inherent in radar systems. Furthermore, target discriminating radar systems based on Doppler processing may evidently not be employed with any success with substantially motionless targets.